C Exercises: Convert a time_t object to calendar time expressed as Coordinated Universal Time

Last update on March 20 2025 12:31:53 (UTC/GMT +8 hours)

6. time_t to UTC Calendar Time

Write a program in C to convert a time_t object to calendar time expressed as Coordinated Universal Time.

Sample Solution:

C Code:

#define __STDC_WANT_LIB_EXT1__ 1 // Macro definition to request C11 library features
#include <stdio.h>
#include <time.h>

int main(void) {
    time_t t = time(NULL); // Get the current calendar time

    // Print the calendar time expressed as Coordinated Universal Time (UTC) and local time
    printf("\nThe calendar time expressed as Coordinated Universal Time is :");
    printf("\nUTC:   %s", asctime(gmtime(&t))); // Convert and print UTC time using gmtime() and asctime()
    printf("local: %s\n", asctime(localtime(&t))); // Convert and print local time using localtime() and asctime()

#ifdef __STDC_LIB_EXT1__
    struct tm buf;
    char str[26];

    // Convert UTC time using gmtime_s() and asctime_s(), and print the formatted string
    asctime_s(str, sizeof str, gmtime_s(&t, &buf));
    printf("UTC:   %s", str);

    // Convert local time using localtime_s() and asctime_s(), and print the formatted string
    asctime_s(str, sizeof str, localtime_s(&t, &buf));
    printf("local: %s", str);
#endif

    return 0;
}

Output:

The calendar time expressed as Coordinated Universal Time is :
UTC:   Thu Aug 03 10:53:03 2017
local: Thu Aug 03 16:23:03 2017

N.B.: The result may varry for your current system date and time.

Flowchart:

For more Practice: Solve these Related Problems:

• Write a C program to convert a time_t object to UTC calendar time and print the result with a "UTC:" prefix.
• Write a C program that displays a time_t object in UTC and also shows the corresponding local time for comparison.
• Write a C program to convert a time_t object to UTC and output a custom formatted string that includes the full date and time.
• Write a C program to display both UTC and local time by converting a time_t object, highlighting the time zone difference.

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